CLARK, THOMAS, M.D. (1801–1867), chemist, was born in 1801 at Ayr. His father was a skilful shipmaster, who sailed all his life to foreign parts without once incurring serious mishap, and his mother a woman of character and ingenuity, who invented the so-called Ayrshire needlework. He went to school at the Ayr Academy until he was fifteen, and was thought a dull boy at first; mathematics, however, drew him out, and he became known as 'the philosopher.' His schooling over, he was placed in the counting-house of Macintosh, the waterproofer, in Glasgow, from which he was transferred after a few years to the St. Rollox chemical works. In 1836 he became lecturer on chemistry at the Glasgow Mechanics' Institution; the same date marks his discovery of the pyrophosphate of soda, a research which Herschel, in his 'Discourse on the Study of Natural Philosophy' (p. 170), singles out for commendation. To improve his footing in the scientific world, he entered as a candidate for the M.D. degree of Glasgow in 1827, completing his curriculum in 1831; in the interval he became apothecary to the infirmary (1829), and wrote several pharmaceutical papers in the 'Glasgow Medical Journal' (Nos. 11, 12, 14). In 1832 he contributed a noteworthy article to the 'Westminster Review' on weights and measures, and in 1834-5 two articles on the patent laws. In 1833 he was elected professor of chemistry in Marischal College and University, Aberdeen, after a competitive examination. He occupied the chair until the fusion of the Marischal College and University with King's College and University in 1860, when he was pensioned; but his career as a teacher practically came to an end in 1843, owing to ill health. In 1848 he had so far recovered as to resume residence in Aberdeen, although not his professorial work. He died on 27 Nov. 1867.

Clark entered vigorously into many controversies, academical, civic, and political, and wrote several pamphlets and many newspaper articles upon them. After he became unable to teach he gave much of his time to the study of English philology and grammar. One of his conclusions is that our modem English was a dialect coexisting with the Anglo-Saxon, but not derived from it. Another of his points was to distinguish in practice between the original (and still colloquial) usage with regard to the relative pronouns 'that' and 'who' or 'which;' the latter he would have restricted to those occasions when the meaning of the relative could be equally well rendered by 'and he' or 'but he,' 'she' or 'it' (see Bain, English Grammar, Preface, and elsewhere). Another of his amateur labours which occupied him many years was to arrange the gospels in parallel columns, and to tabulate the various Greek readings of the first three; by this work, which was withheld from publication by his executors, it is stated by his biographer. Dr. Alexander Bain, that 'no such elaborateness of inquiry was ever shown in any learned research.' Nearly at the end of his life Clark emerged for a moment from his privacy to take his seat in the university court of St. Andrews, as assessor appointed by the rector, Mr. J. S. Mill, who had known and esteemed him for many years.

Clark is best known by his water tests and by his process for softening chalk waters. His soap test (for hardness) made a new departure in the analysis of waters, and was speedily enforced by the government in the examination of all waters proposed to be supplied to towns. His other great invention was the process of softening waters rendered hard by the presence of bicarbonate of lime in solution, a process that Thomas Graham has been known to speak of as 'the most consummate example of applied science in the whole circle of the arts. If forty gallons of water in which caustic lime has been dissolved be added to five hundred gallons of hard water, or water holding bicarbonate of lime in solution, the second molecule of carbonic acid in the latter leaves it to combine with the caustic lime, the result being that all the lime (two pounds) is deposited in the form of the insoluble carbonate, and the 540 gallons of water remain clear and soft. Water so softened would require only one-third the quantity of soap to make a lather ; also there would be no fur on the surface of boilers. The advantage of Clark's process over other softening processes is that no derivative compounds remain behind in the water. 'This character,' says Clark, 'is as fortunate as it is rare in chemical processes.' Another advantage is that the quantity of organic matter in the water is greatly reduced by the precipitation of the chalk, the water in large bulk having the natural pure blue colour of uncontaminated water, the process is somewhat expensive, from the numher of reservoirs required; but the cost of the caustic lime is more than balanced by the high price got for the chalk thrown down. Although the process was favourably reported on to the government in 1851 by Graham, Miller, and Hoffmann, it was opposed by the metropolitan water companies, and has been adopted at only a few places. The following is a complete list of the larger works: Plumstead, 1854 (absorbed in 1861 by the Kent Water Company, who do not soften); Caterham, 186l; Chiltern Hills, 1807 (supplying Aylesbury, Tring, &c.); Canterbury, 1869; and Colne Valley, 1876 (supplying the district as far as Harrow, Hendon, and Edgware, from the reservoirs at Bushey). The process is also in use at private establishments, such as Castle Howard, Mentmore, Henley Park Place, and the Herbert Hospital. Clark's sanguine forecast was, 'The process is of such utility and such necessity to London that it will be in operation as long as London last.'

[Biographical Memoir of Dr. Thomas Clark, by Alexander Bain, in the Transactions of the Aberdeen Philosophical Society, 1840-84.]